COMPLEXOMETRIC
TITRATION
An application method of
Inorganic Pharmaceutical Analysis
Lecturer : Dr. Tutus Gusdinar
Pharmacochemistry Research Group
School of Pharmacy
INSTITUT TEKNOLOGI BANDUNG
• Complexometric titration is a type of
titration based on complex formation
between the analyte and titrant.
• Complexometric titrations are particularly
useful
f l ffor determination
d t
i ti off a mixture
i t
off
different metal ions in solution. An
indicator with a marked color change is
usually used to detect the end-point
end point of the
titration.
Any complexation reaction can in theory be applied
as a volumetric technique provided that :
• the reaction reaches equilibrium rapidly following
each addition of titrant.
• interfering situations do not arise (such as stepwise
formation of various complexes resulting in the
presence of more than one complex in solution in
significant concentration during the titration
process).
process)
• an complexometric indicator capable of locating
equivalence point with fair accuracy is available
I practice,
In
ti
the
th use off EDTA as a tit
titrantt iis well
ll established.
t bli h d
Complexometric titration with EDTA
Ethylenediamminetetraacetic acid, has four carboxyl groups
and two amine groups that can act as electron pair donors
donors, or
Lewis bases. The ability of EDTA to potentially donate its six
lone pairs of electrons for the formation of coordinate covalent
bonds to metal cations makes EDTA a hexadentate ligand.
However in practice EDTA is usually only partially ionized
However,
ionized, and
thus forms fewer than six coordinate covalent bonds with metal
cations Disodium EDTA
cations.
EDTA, commonly used in the standardization
of aqueous solutions of transition metal cations, only forms four
coordinate covalent bonds to metal cations at pH values less
than or equal to 12 as in this range of pH values the amine
groups remain protonated and thus unable to donate electrons
to the formation of coordinate covalent bonds.
In analytical chemistry the shorthand "Na2H2Y" is typically
used to designate disodium EDTA. This shorthand can be
used to designate any species of EDTA. The "Y" stands for
the EDTA molecule, and the "Hn" designates the number of
acidic protons bonded to the EDTA molecule.
EDTA forms an octahedral complex with most 2+ metal
cations M2+, in aqueous solution
cations,
solution. The main reason that EDTA
is used so extensively in the standardization of metal cation
solutions is that the formation constant for most metal cationEDTA complexes is very high, meaning that the equilibrium
for the reaction :
2 + H Y → MH Y + 2H+
M2+
4
2
lies far to the right. Carrying out the reaction in a basic buffer
solution removes H+ as it is formed
formed, which also drives the
reaction to the right. For most purposes it can be considered
that the formation of the metal cation-EDTA complex
p
g
goes to
completion, and this is chiefly why EDTA is used in titrations /
standardizations of this type.
To carry outt metal
T
t l cation
ti titrations
tit ti
using
i EDTA it is
i almost
l
t
always necessary to use a complexometric indicator, usually
an organic dye such as Fast Sulphon Black,
Black Eriochrome
Black T, Eriochrome Red B or Murexide, to determine when
the end point has been reached.
These dyes bind to the metal cations in solution to form
colored complexes.
comple es However,
Ho e er since EDTA binds to metal
cations much more strongly than does the dye used as an
indicator the EDTA will displace the dye from the metal
cations as it is added to the solution of analyte.
A color change in the solution being titrated indicates that all
of the dye has been displaced from the metal cations in
solution and that the endpoint has been reached
solution,
reached.
Molecular structure of EDTA
• Complexometric
p
titration has made it
possible for man to be exposed to an
advanced method of titration which not only
enables us to analyze more ions, but also do
them in very small quantities
quantities.
• We’ve to be aware of the effects of p
pH on
the titration method. Complex ion titration is
possible in very minute quantities
quantities. The
biological use of complexometric titration
seems to involve an advanced method of
this kind of titration; and we can learn its
application
li ti on liliving
i cells.
ll
The general shape of titration curves obtained by titrating 10.0 mL of a
0 01 solution off a metal ion M with a 0
0.01M
0.01
01 M EDTA solution. The apparent
stability constants of various metal-EDTA complexes are indicated at the
extreme right of the curves
curves. It is evident that the greater the stability
constant, the sharper is the end point provided the pH is maintained
constant.
In acid-base titrations the end point is generally detected by a pH-sensitive
i di t IIn th
indicator.
the EDTA tit
titration
ti a metal
t l ion
i
sensitive
iti indicator
i di t ( metal
t l
indicator or metal-ion indicator) is often employed to detect changes of pM.
Such indicators (which contain types of chelate groupings and generally
possess resonance systems typical of dyestuffs) form complexes with
specific metal ions, which differ in colour from the free indicator and produce
a sudden colour change at the equivalence point. The end point of the
titration can also be evaluated by other methods including potentiometric,
amperometric,
t i and
d spectrophotometric
t h t
t i techniques.
t h i
A. Direct titration. The solution containing the metal ion to be determined
is buffered to the desired p
pH ((e.g.
g to p
pH = 10 with NH:-aq.
q NH,)
,) and
titrated directly with the standard EDTA solution. It may be necessary to
prevent precipitation of the hydroxide of the metal (or a basic salt) by the
addition of some auxiliary complexing agent
agent, such as tartrate or citrate or
triethanolamine.
At the equivalence point the magnitude of the concentration of the metal
ion being determined decreases abruptly
abruptly. This is generally determined by
the change in colour of a metal indicator or by amperometric, spectrophotometric, or potentiometric methods.
B. Back-titration. Many metals cannot, for various reasons, be titrated
directly; thus they may precipitate from the solution in the pH range
necessary for the titration, or they may form inert complexes, or a suitable
metal indicator is not available. In such cases an excess of standard
EDTA solution is added, the resulting
g solution is buffered to the desired
pH, and the excess of the EDTA is back-titrated with a standard metal ion
solution; a solution of zinc chloride or sulphate or of magnesium chloride
or sulphate is often used for this purpose.
The end point is detected with the aid of the metal indicator which
responds to the zinc or magnesium ions introduced in the back-titration.
C. Replacement or substitution titration. Substitution titrations may be
used for metal ions that do not react (or react unsatisfactorily) with a metal
indicator, or for metal ions which form EDTA complexes
p
that are more
stable than those of other metals such as magnesium and calcium. The
metal cation Mn+ to be determined may be treated with the magnesium
complex of EDTA, when the following reaction occurs :
The amount of magnesium ion set free is equivalent to the cation present
and can be titrated with a standard solution of EDTA and a suitable metal
indicator.
An iinteresting
A
i application
li i iis the
h titration
i i off calcium.
l i
IIn the
h di
direct titration
i i
of calcium ions, solochrome black gives a poor end point; if magnesium is
present, it is displaced from its EDTA complex by calcium and an
improved end point results
D. Alkalimetric titration.
When a solution of disodium ethylenediaminetetraacetate, Na2H2Y, is
added to a solution containing metallic ions
ions, complexes are formed with
the liberation of two equivalents of hydrogen ion:
The hydrogen ions thus set free can be titrated with a standard solution of
sodium hydroxide using an acid-base indicator or a potentiometric end
point;; alternatively,
p
y, an iodate-iodide mixture is added as well as the EDTA
solution and the liberated iodine is titrated with a standard thiosulphate
solution.
The solution of the metal to be determined must be accurately neutralised
before titration; this is often a difficult matter on account of the hydrolysis
of many salts,
salts and constitutes a weak feature of alkalimetric titration
titration.
E. Miscellaneous methods. Exchange reactions between the tetracyanonickelate(
I1) ion [Ni(CN),I2- (the potassium salt is readily prepared) and the element to be
determined whereby nickel ions are set free
determined,
free, have a limited application
application. Thus silver
and gold, which themselves cannot be titrated complexometrically, can be
determined in this way.
These reactions take place with sparingly soluble silver salts, and hence provide
a method for the determination of the halide ions Cl-, Br-, 1-, and the thiocyanate
ion SCN-. The anion is first precipitated as the silver salt, the latter dissolved in a
solution of [[Ni(CN),I2-,
( ), , and the equivalent
q
amount of nickel therebyy set free is
determined by rapid titration with EDTA using an appropriate indicator (murexide,
bromopyrogallol red).
Fluoride may be determined by precipitation as lead chlorofluoride, the precipitate being
dissolved in dilute nitric acid and, after adjusting the pH to 5-6, the lead is titrated with EDTA
using xylenol orange indicator.
Sulphate may be determined by precipitation as barium sulphate or as lead sulphate.
sulphate The
precipitate is dissolved in an excess of standard EDTA solution, and the excess of EDTA is
back-titrated with a standard magnesium or zinc solution using solochrome black as indicator.
Phosphate may be determined by precipitating as Mg(NH,) PO,, 6H2 O, dissolving the
precipitate in dilute hydrochloric acid, adding an excess of standard EDTA solution, buffering at
pH = 10, and back-titrating with standard magnesium ion solution in the presence of
solochrome black.
TlTRATlON OF MIXTURES,,
SELECTIVITY, MASKING AND
DEMASKING AGENTS
EDTA is a very unselective reagent because it complexes with numerous
doubly, triply and quadruply charged cations. When a solution containing
two cations which complex with EDTA is titrated without the addition of a
complex-forming
l f
i iindicator,
di t and
d if a tit
titration
ti error off 0
0.1
1 per centt iis
permissible, then the ratio of the stability constants of the EDTA complexes
of the two metals M and N must be such that KM/KN > 106 if N is not to
interfere with the titration of M.
Strictly, of course, the constants KM and KN considered in the above
expression should be the apparent stability constants of the complexes. If
complex-forming indicators are used, then for a similar titration error
KM/KN > 108.
The following procedures will help to increase the selectivity :
(a) Suitable control of the pH of the solution.
This, of course, makes use of the different stabilities of metal-EDTA
complexes. Thus bismuth and thorium can be titrated in an acidic
solution (pH = 2) with xylenol orange or methylthymol blue as indicator
and most divalent cations do not interfere.
A mixture of bismuth and lead ions can be successfully titrated by first
titrating the bismuth at pH 2 with xylenol orange as indicator, and then
adding hexamine to raise the pH to about 5, and titrating the lead.
( ) Use of masking
(b)
g agents.
g
Masking may be defined as the process in which a substance, without
physical separation of it or its reaction products, is so transformed that
it does not enter into a particular reaction. Demasking is the process in
which the masked substance regains its ability to enter into a particular
reaction.
reaction
By the use of masking agents, some of the cations in a mixture can often
be 'masked' so that they
y can no longer
g react with EDTA or with the
indicator. An effective masking agent is thecyanide ion; this forms stable
cyanide complexes with the cations of Cd, Zn, Hg(II), Cu, Co, Ni, Ag, and
the platinum metals, but not with the alkaline earths, manganese, and
lead :
It is therefore possible to determine cations such as Ca2+, Mg2+, Pb2+,
and Mn2+ in the presence of the above-mentioned metals by masking
with an excess of p
potassium or sodium cyanide.
y
A small amount of iron
may be masked by cyanide if it is first reduced to the iron(II) state by the
addition of ascorbic acid. Titanium(IV), iron(III), and aluminium can be
masked with triethanolamine; mercury with iodide ions; and aluminium,
iron(III), titanium(IV), and tin(II) with ammonium fluoride (the cations of
the alkaline-earth
alkaline earth metals yield slightly soluble fluorides)
fluorides).
y be transformed into a different oxidation state:
Sometimes the metal may
thus copper(II) may be reduced in acid solution by hydroxylamine or
ascorbic acid. After rendering ammoniacal, nickel or cobalt can be
titrated using, for example, murexide as indicator without interference
from the copper, which is now present as Cu(I). Iron(III) can often be
similarly masked by reduction with ascorbic acid.
acid
( ) Selective demasking.
(c)
g
The cyanide complexes of zinc and cadmium may be demasked with
formaldehyde-acetic acid solution or, better, with chloral hydrate :
The use of masking and selective demasking agents permits the
successive titration of many metals. Thus a solution containing Mg, Zn,
and Cu can be titrated as follows:
1 Add excess off standard
1.
t d d EDTA and
db
back-titrate
k tit t with
ith standard
t d d Mg
M
solution using solochrome black as indicator. This gives the sum of al1
the metals present.
2. Treat an aliquot portion with excess of KCN (Poison !) and titrate as
before. This gives Mg only.
3. Add excess of chloral hydrate
y
(or
( of formaldehyde-acetic
y
acid solution,
3:1) to the titrated solution in order to liberate the Zn from the cyanide
complex, and titrate until the indicator turns blue. This gives the Zn only.
The Cu content may then be found by difference.
(d) Classical separation. These may be applied if they are not tedious; thus the
following precipitates may be used for separations in which
which, after being
re-dissolved, the cations can be determined complexometrically : CaC2O4, nickel
dimethylglyoximate, Mg(NH2)P04,6H2O, and CuSCN.
(e) Solvent extraction.
extraction This is occasionally of value
value. Thus zinc can be separated
from copper and lead by adding excess of ammonium thiocyanate solution and
extracting the resulting zinc thiocyanate with 4-methylpentan-2-one (isobutyl
methyl ketone); the extract is diluted with water and the zinc content determined
with EDTA solution.
(f) Choice of indicators. The indicator chosen should be one for which the
formation of the metal-indicator complex
p
is sufficiently
y rapid
p to p
permit
establishment of the end point without undue waiting, and should preferably
be reversible.
(g) Removal of anions. Anions, such as orthophosphate, which can interfere in
complexometric titrations may be removed using ion exchange resins. For the
use of ion exchange resins in the separation of cations and their subsequent
EDTA titration.
(h) 'Kinetic masking'. This is a special case in which a metal ion does not
effectively enter into the complexation reaction because of its kinetic inertness.
Thus the slow reaction of chromium(III) with EDTA makes it possible to titrate other
metal
t l ions
i
which
hi h reactt rapidly,
idl without
ith t iinterference
t f
ffrom C
Cr(III);
(III) this
thi is
i illustrated
ill t t d b
by
the determination of iron(III) and chromium(III) in a mixture.
Dyestuffs which form complexes with specific metal
pM values;; 1:1cations can serve as indicators of p
complexes (metal : dyestuff = 1:1) are common,
but 1:2-complexes and 2:1-complexes also occur.
The metal ion indicators, like EDTA itself, are chelating
agents; this implies that the dyestuff molecule possesses
several ligand atoms suitably disposed for coordination
with a metal atom.
They can, of course, equally take up protons, which also
produces
d
a colour
l
change;
h
metal
t l iion iindicators
di t
are
therefore not only pM but also pH indicators.
METAL ION INDICATORS
The success of an EDTA titration depends upon the precise determination of the end point. The
most common procedure utilises metal ion indicators. The requisites of a metal ion indicator for
use in the visual detection of end points include :
(a) The colour reaction must be before the end point, when nearly all the metal ion is
complexed with EDTA
EDTA, the solution is strongly coloured
coloured.
(b) The colour reaction should be specific or selective.
(c) The metal-indicator complex must possess sufficient stability, otherwise, due to
di
dissociation,
i ti
a sharp
h
colour
l
change
h
iis nott attained.
tt i d Th
The metal-indicator
t l i di t complex
l
must, however, be less stable than the metal-EDTA complex to ensure that, at
the end point, EDTA removes metal ions from the metal indicator-complex. The
change in equilibrium from the metal indicator complex to the metal
metal-EDTA
EDTA
complex should be sharp and rapid.
(d) The colour contrast between the free indicator and the metal-indicator complex
should be readily observed
observed.
(e) The indicator must be very sensitive to metal ions (i.e. to pM) so that the colour
change occurs as near to equivalence point as possible.
(f) The above requirements must be fulfilled within the pH range at which the titration
is performed.
Theory of the visual use of
metal
t l ion
i indicators
i di t
Discussion will be confined to the more common 1:1-complexes. The use of a metal
ion indicator in an EDTA titration may be written as:
This reaction will proceed iff the metal-indicator complex M-In is less stable than the
metal-EDTA complex M-EDTA. The former dissociates to a limited extent, and during
the titration the free metal ions are progressively complexed by the EDTA until
ultimately
lti t l the
th metal
t l is
i displaced
di l
d ffrom th
the complex
l M
M-In
I to
t leave
l
the
th free
f
indicator
i di t
(In). The stability of the metal-indicator complex may be expressed in terms of the
formation constant (or indicator constant) KIn:
The indicator colour change is affected by the hydrogen ion concentration
of the solution,
solution and no account of this has been taken in the above expression
for the formation constant. Thus solochrome black, which may be written as
H2In-, exhibits the following acid-base behaviour :
Practical considerations
1) Adjustment of pH.
For many EDTA titrations
F
tit ti
the
th pH
H off the
th solution
l ti is
i extremely
t
l critical;
iti l
often limits of 11 unit of pH, and frequently limits of 10.5 unit of pH
must be achieved for a successful titration to be carried out
out. To achieve
such narrow limits of control it is necessary to make use of a pH meter
while adjusting the pH value of the solution, and even for those cases
where the latitude is such that a pH test-paper can be used to control
the adjustment of pH, only a paper of the narrow range variety should
b used.
be
d
2) Concentration of the metal ion to be titrated.
Most titrations are successful with 0.25 millimole of the metal ion
concerned in a volume of 50
50-150mL
150mL of solution.
solution If the metal ion
concentration is too high, then the end point may be very difficult to
discern, and if difficulty
y is experienced
p
with an end p
point then it is
advisable to start with a smaller portion of the test solution, and to dilute
this to 100-150 mL before adding the buffering medium and the
indicator, and then repeating the titration.
3) Amount of indicator.
The addition of too much indicator is a fault which must be guarded
against: in many cases the colour due to the indicator intensifies
considerably
y during
g the course of the titration,, and further,, manyy
Indicators exhibit dichroism, i.e. there is an intermediate colour change
one to two drops before the real end-point. Thus, for example, in the
titration of lead using xylenol orange as indicator at pH = 6,
6 the initial
reddish-purple colour becomes orange-red, and then with the addition of
one or two further drops of reagent, the solution acquires the final lemon
yellow colour
colour. This end point anticipation,
anticipation which is of great practical
value, may be virtually lost if too much of the indicator is added so that
the colour is too intense. In general, a satisfactory colour is achieved by
th use off 30-50
the
30 50 mg off a 1 per centt solid
lid mixture
i t
off th
the iindicator
di t in
i
potassium nitrate.
4) Attainment of the end point.
In many EDTA titrations the colour change in the neighbourhood of
the end p
point may
y be slow. In such cases,, cautious addition of the
titrant coupled with continuous stirring of the solution is advisable;
the use of a magnetic stirrer is recommended.
Frequently, a sharper end point may be achieved if the solution is
warmed to about 40 OC.
Titrations with CDTA are always slower in the region of the end point
than the corresponding EDTA titrations.
5) Detection
D t ti off th
the colour
l
change.
h
With all of the metal ion indicators used in complexometric
p
titrations,,
detection of the end point of the titration is dependent upon the
recognition of a specified change in colour; for many observers this
can be a difficult task
task, and for those affected by colour blindness it
may be virtually impossible. These difficulties may be overcome by
replacing the eye by a photocell which is much more sensitive, and
eliminates the human element
element.
To carry out the requisite operations it is necessary to have available
a colorimeter
l i t or a spectrometer
t
t in
i which
hi h th
the cellll compartment
t
t is
i
large enough to accommodate the titration vessel (a conical flask or a
tall form beaker). A simple apparatus may be readily constructed in
which light passing through the solution is first allowed to strike a
suitable filter and then a photocell; the current generated in the latter
g
is measured with a galvanometer.
6) Alternative methods of detecting the end
point.
point
In addition to the visual and spectrophotometric detection of end
points
i t iin EDTA titrations
tit ti
with
ith th
the aid
id off metal
t l iion iindicators,
di t
th
the
following methods are also available for end point detection :
• Potentiometric titration using a mercury electrode.
• Potentiometric titration using a selective ion electrode responsive
to the ion being
g titrated.
• Potentiometric titration using a bright platinum-saturated calomel
electrode system; this can be used when the reaction involves two
different oxidation states of a given metal
metal.
• By amperometric titration.
• By coulometric analysis.
• By conductimetric titration.
titration
Some applications
in human life
Complexometric and Medicine
Complexometric is widely used in the medical
industry because of the microliter-size
microliter size sample
involved. The method is efficient in research
related to the biological cell.
• Ability to titrate the amount of ions available
in a living cell.
• Ability to introduce ions into a cell in case of
deficiencies.
Complexometric tiration involves the treatment of
complex ions such as magnesium, calcium,
copper, iron, nickel, lead and zinc with EDTA as
the complexing agent.
The titration of EDTA depends on pH stability.
Performed at high (basic) pH, i.e. pH 10 for Ca+2 or Mg+2
High Formation Constant
Constant, Kf
Diffusional Microburette : used to deliver the EDTA in
microscopic droplets, as low as 6 fmol/s, observed under
p
microscope.
Complexometric
p
Titration and
Water Hardness
Complexometric titration is an efficient method for
g the level of hardness of water. Caused by
y
determining
accumulation of mineral ions, pH of water is increased.
The Kf during the titration of hard water is reduced because
of the reduced amount of EDTA added.
Softening of hard water is done by altering the pH of the
water reducing the concentration of the metal ions present
present.
Could be performed in two phases : Basic pH for ions with
g Kf e.g.
g Ca+2 and Mg
g+2
high
Zinc in Water
The traces of zinc in water can be
determined with complexometric titration
titration.
Results from these analysis show that
5 bottles of water daily for minimum zinc
q
quantity.
y
The amount of Zn in soil water has been
increased by bio-activities
bio activities.
The EDTA could also be performed for complex
metal ions at lower pH.
pH
Zinc has a low Kf = 3.16 x 10-17
Zn+2 can be titrated in acidic pH 5
5.5
5
Zinc in Food
Certain foods contain zinc in small
amounts. Oysters and other seafood,
meat, liver, eggs, milk and brewer's yeast.
Zinc deficiency is rare
rare. It occurs when
there is :
• Excess alcohol
• Excess exercisec as sweat depletes zinc stores
• A strict vegetarian diet
Zinc in the Body
• Reduces zinc-containing
enzyme, carbonic
anhydrase in red blood
cells.
• Guard against infections
• Repair wounds
• Brain development
• Smell sensation
• Beef contains the most
amount of vitamins
• Vegetarians
V
t i
iin US gett
10-30% less zinc than
non-vegetarians
non
vegetarians.
• The average adult man is
gets
ge
s abou
about 90% o
of the
e
recommended level of
zinc; women, 25%.
Kids are also below the
recommended level.
The END